1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a method of manufacturing a liquid crystal display device in which alignment process is performed to obtain a desired twist angle.
2. Discussion of the Related Art
Generally, a twisted-nematic(TN)-LCD device used as a large area, high quality LCD device has a liquid crystal alignment film arranged inside a transparent electrode to directly adjoin liquid crystal molecules. On the interface between the liquid crystal alignment film and the liquid crystal molecules, the liquid crystal is aligned by a unidirectional alignment of the liquid crystal molecules and a pre-tilt angle of the liquid crystal alignment film and the liquid crystal molecules. The unidirectional alignment is performed in such a manner that the liquid crystal alignment film is stretched unidirectionally.
A related art rubbing method used to perform a unidirectional stretching process on the liquid crystal alignment film is performed in such a manner that a substrate coated with a high polymer is rubbed with a cloth. Since the related art rubbing method enables a large area and high speed process, it is industrially and widely used.
If the substrate is rubbed by the related art rubbing method, micro grooves are uniformly formed in the alignment film. The micro grooves are arranged in parallel to the liquid crystal molecules to minimize elastic deformation energy. However, in the related art rubbing method, types of the micro grooves formed in the alignment film are different according to the intensity of friction between an alignment cloth and the alignment film. Therefore, the liquid crystal molecules are unevenly arranged, thereby causing phase distortion and light scattering. The phase distortion and light scattering may significantly act on the performance of the LCD.
In the related art rubbing method, a value of a pre-tilt angle is susceptible to deposition or rubbing condition. Accordingly, to realize the related art rubbing method, sufficient consideration is required. Moreover, since the related art rubbing method is performed in such a manner that the high polymer film is rubbed with the alignment cloth, fine dust or electrostatic discharge (ESD) may occur. In other words, the dust causes serious problems in forming a high resolution pixel electrode or a thin film transistor (TFT) by repeatedly performing deposition, exposure and etching processes. Furthermore, local discharge may damage the alignment film, or cause short of a transparent electrode or the TFT, or destruct static electricity. Besides, the alignment film for TFT requires high voltage retention ratio capable of retaining charges for a long time.
Particularly, in the TN-LCD, light transmittivity is symmetrically distributed for a viewing angle in left and right direction, but asymmetrically distributed for a viewing angle in up and down direction. Image is shifted for the viewing angle in up and down direction, thereby causing the viewing angle to be narrowed.
To supplement anisotropic characteristic of the liquid crystal, a multi-domain TN-LCD such as a TDTN(two domain TN)-LCD and DDTN(domain divided TN) are provided. The process of the multi-domain TN-LCD is characterized by photolithography and rubbing. That is to say, to form domains within each pixel in opposite directions or different alignment directions, photolithography and rubbing processes are twice required. Furthermore, four-domain TN-LCD may be provided.
However, the multi-domain TN-LCD has several problems. That is, the two domain TN-LCD has a viewing angle of ±25° at most in up and down direction within the range of a contrast ratio of 10 or greater. The four domain TN-LCD has a viewing angle of ±40° at most in up and down direction and left and right direction. Moreover, the multi-domain TN-LCD has a complicated manufacturing process.
To solve the problems of the rubbing method, a photo-alignment method is provided. The photo-alignment method is based on a photo-polymerization of the alignment film by irradiation of light. In the photo-alignment method, the alignment direction is determined by irradiating ultraviolet rays two times.
Also, in the photo-alignment method, a PVCN(polyvinylclinnamate) based material is used as a photo-alignment film, and a pre-tilt direction on a surface of the alignment film is determined by irradiating ultraviolet rays to the alignment film vertically and slantingly. At this time, the pre-tilt angle of a surface of the alignment film is determined by varying the alignment film and an irradiation angle of the ultraviolet rays.
However, in case where the photo-alignment film has low isotropic characteristic and anchoring energy, the photo-alignment method has a problem. That is, the liquid crystal is implanted to the alignment film, a director of the liquid crystal molecule adjacent to the actual alignment film is not coincident with alignment direction. Accordingly, the alignment film has a twist angle different from a desired twist angle.
The difference between the director of the liquid crystal molecule intended in the alignment direction and the director of the actual liquid crystal molecule fails to obtain normally white mode or normally black mode and deteriorates contrast ratio.